Hydrocarbon Processing - December 2021 - 42

Process Optimization
Product to recovery
Inert gas
Spent air
Regenerator
Reactive
Air
Separator/stripper
the regenerator. The use of a catalyst cooler commonly
used in modern, large-scale FCCUs is not practical in the
CFFB reactor side, or often even for the regenerator.
* Fast separation of gas and solid at the exit of the riser
in the FCCU is critical to minimizing gas yield. This
separation is not as important in most, if not all, CFFBs.
* CFFBs require special considerations on how the reactor
exit is connected to cyclones due to the high cost of the
catalyst.
* Average density in the FCC riser is about 80 kg/m3
(5 lb/ft3
than 160 kg/m3 (10 lb/ft3
). In the CFFB, riser density is typically more
). The CFFB reactor also
operates at significantly lower velocity than the FCC
riser. Moving such a dense mixture vertically up a
tall riser at low velocity is not an easy task.
Butane
Secunda reactor
Recycle gas
FIG. 1. Examples of CFFBs.
CFFBs have been assessed both in concept and through
demo-scale production plants for the conversion of natural
gas directly to olefins by most major gas field producers. So
far, however, the economics have not justified further development-the
main barrier being process yield.
CFFB vs FCC. A CFFB has many design aspects similar to
that of an FCC, which is referred to here as a circulating fast bed
(CFB). A Group A powder chemical synthesis CFFB will also
contain a reactor that operates as a riser, but it should not be designed
using FCC design guidelines for the following reasons:
* Use of oxygen gas in the reactor side of the CFFB poses
safety considerations with respect to the design of the
reactor in the event of loss of solids circulation. This
concern is not present in FCC. Deflagration may occur
in the CFFB reactor due to a drop in solids holdup
caused by loss of solid circulation.
* Chemical synthesis catalyst may cost up to $110/kg
($50/lb) with a life of many years compared to less
than $8/kg and a few months of life for FCC catalyst.
In CFFB, the unit is typically designed to minimize
attrition. In FCC, attrition of the catalyst is not a
major design concern.
* The diameter ratio of regenerator to standpipe in
modern FCCUs is very large. The ratio in a CFFB with
the same size standpipe is significantly smaller as coke
production is much less in CFFBs, or not present
at all. Most regenerator standpipe entry designs used
in FCC do not translate well to a CFFB.
* FCC uses atomized liquid feed in the reactor. A CFFB for
chemical synthesis is likely to use gas feed, which creates
challenges on how to mix the feed with circulated catalyst
in the reactor bottom, especially if the reaction involves
the use of oxygen (gas or solid phase).
* The FCC reactor is endothermic. The FCC regenerator is
exothermic and may or may not require heat removal. The
CFFB may require heat removal in both the reactor and
42 DECEMBER 2021 | HydrocarbonProcessing.com
Regenerator temperature control options. Approaches to
controlling the temperature of the regenerator vary depending
on the type of process being considered, ranging from external
catalytic coolers for FCC to internal cooling coils or other approaches
for chemical synthesis (TABLE 1).
Riser solid holdup. FIG. 2 may be used to estimate CFB riser solid
holdup as a function of velocity and solid flux and applies to solid
particle density of approximately 1,100 kg/m3
-1,750 kg/m3
/sec.
. The
vol% of solids can be obtained from the graph and multiplied by
particle density. Commercial CFFBs typically operate with solids
holdup above 10 vol% and fluxes over 300 kg/m2
Scale-up. The author's experience suggests that there are no
hydrodynamic reasons to limit the capacity of a fluid bed using
Group A powder that has proper solid size distribution, operating
velocity and design of internals. The factors that limit CFFB
unit capacities (reactor size) are typically as follows:
* Plot space: The structure space (width, length and
especially height) needed to connect three vessels
(reactor, stripper, regenerator) together in a CFFB loop
becomes excessively large as equipment size is increased.
* Cost factors: Shop fabrication is usually cheaper than
field fabrication. Vessel diameters are, therefore, limited
to about 4.5 m (15 ft) due to over-the-road accessibility
with shop fabrication. Since FFBs have a single vessel,
the structure height changes only a little with the size of
the reactor. As a result, single-train field-fabricated FFB
reactors of 7 m (23 ft) and larger are commonly used.
The pilot plant, demo plant or first commercial plant of a
new process does not need to be optimal. The plant should
work reliably and prove that the process concept is commercially
viable. Optimization can be achieved by tweaking the
second commercial plant. Admittedly, this approach may require
some calculated risk. It is the faster approach to demonstrating
a new Group A-based fluid bed process (CFFB, CFB
or FFB). The alternative is to resolve all issues, thereby extending
" time to market, " during which period engineering and research
hours are spent increasing the " cost to market. "
* FCC catalyst pores contain high-boiling-point
components that are difficult to strip. CFFB catalysts
typically does not have this issue, so stripping in the
CFFB is generally easier.
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